Natural killer (NK) cells are large granular lymphocytes that spontaneously lyse certain tumor cells and cell lines. We have previously shown that NK cells respond specifically to target cells by stimulating the polyphosphoinositide (PPI) signaling pathway, leading to the generation of inositol phosphates and a rise in intracellular calcium, and we showed that these signals are associated with cytotoxicity. By studying an NK-like rat leukemia cell line, RNK- 16, we have identified two previously unknown surface molecules that can stimulate the PPI pathway. Our proposed studies will examine in detail the function and regulation of one of these, gp42. gp42 is expressed on RNK-16 cells by anchoring to glycosylphosphatidylinositol (GPI): it is the only known GPI-anchored molecule on lymphocytes that can signal in the absence of the T cell receptor. The first goal of our proposed studies is to define the structural requirements for signaling by gp42. A second aspect of our studies involves the restricted expression of gp42 on rat leukocytes. gp42 is not expressed by resting NK cells, but it is uniquely induced on NK cells in response to interleukin-2 (IL-2). It is the only activation antigen that is specific for NK cells. The second goal of our proposed work is to define the genetic regulatory elements that restrict expression of gp42 to activated NK cells. The third goal of our proposal is to clone the cDNA for human gp42 and to use this in developing monoclonal antibodies for the study of human gp42.